Fuzzy Logic - Introduction

S.G.Sanjeevi

• fuzzy set A • A = {(x, µA(x))| x Є X} where µA(x) is called

the membership function for the fuzzy set A. X is referred to as the universe of discourse.

• The membership function associates each element x Є X with a value in the interval [0,1].

Fuzzy sets with a discrete universe

• Let X = {0, 1, 2, 3, 4, 5, 6} be a set of numbers of children a family may possibly have.

• fuzzy set A with “sensible number of children in a family” may be described by

• A = {(0, 0.1), (1, 0.3), (2, 0.7), (3, 1), (4, 0.7), (5, 0.3), (6, 0.1)}

Fuzzy sets with a continuous universe

• X = R+ be the set of possible ages for human beings.

• fuzzy set B = “about 50 years old” may be expressed as

• B = {(x, µB(x)∣x Є X}, where

• µB(x) = 1/(1 + ((x-50)/10)4

We use the following notation to describe fuzzy sets.

• A = Σ xi Є X µA(xi)/ xi, if X is a collection of discrete objects,

• A = ∫X µA(x)/ x, if X is a continuous space.

age

1.0

µ

0.0

young middle aged old

• Support(A) is set of all points x in X such that

• {(x∣ µA(x) > 0 }

• core(A) is set of all points x in X such that• {(x∣ µA(x) =1 }

• Fuzzy set whose support is a single point in X with µA(x) =1 is called fuzzy singleton

• Crossover point of a fuzzy set A is a point x in X such that

• {(x∣ µA(x) = 0.5 }

• α-cut of a fuzzy set A is set of all points x in X such that

• {(x∣ µA(x) ≥ α }

• Convexity µA(λx1 + (1-λ)x2 ) ≥min(µA( x1), µA( x2)). Then A is convex.

• Bandth width is |x2-x1| where x2 and x1 are crossover points.

• Symmetry µA( c+x) = µA( c-x) for all x Є X. Then A is symmetric.

• Containment or Subset: Fuzzy set A is contained in fuzzy set B (or A is a subset of B) if A(x) B(x) for all x.

• Fuzzy Intersection: The intersection of two fuzzy sets A and B , AB or A AND B is fuzzy set C whose membership function is specified by the

C(x) = AB = min(A(x), B(x)) = A(x) ٨ B(x).

• Fuzzy Union : The union of two fuzzy sets A and B written as AB or A OR B is fuzzy set C whose membership function is specified by the

C(x) = AB = max(A(x), B(x)) = A(x) ٧ B(x).

• Fuzzy Complement: The complement of A denoted by Ā or NOT A and is defined by the membership function Ā (x) = 1 - A(x).

Membership functions of one dimension

• A triangular membership function is specified by three parameters {a, b, c}:

• Triangle(x; a, b, c) = 0 if x a;• = (x-a)/(b-a) if a x b;• = (c-b)/(c-b) if b x c;• = 0 if c x.

A trapezoidal membership function is specified by four parameters {a, b, c, d} as follows:

• Trapezoid(x; a, b, c, d) = 0 if x a;• = (x-a)/(b-a) if a x b;• = 1 if b x c;• = (d-x)/(d-c) 0 if c x d;• = 0, if d x.

A sigmoidal membership function is specified by two parameters {a, c}:

• Sigmoid(x; a, c) = 1/(1 + exp[-a(x-c)]) where a controls slope at the crossover point x = c.

• These membership functions are some of the commonly used membership functions in the fuzzy inference systems.

Membership functions of two dimensions

• One dimensional fuzzy set can be extended to form its cylindrical extension on second dimension

• Fuzzy set A = “(x,y) is near (3,4)” is • µA(x,y) = exp[- ((x-3)/2)2 -(y-4)2 ]• µA(x,y) = exp[- ((x-3)/2)2 ] exp -(y-4)2 ]• =gaussian(x;3,2)gaussian(y;4,1)• This is a composite MF since it can be

decomposed into two gaussian MFs

Fuzzy intersection and Union

AB = T(A(x), B(x)) where T is T-norm operator. There are some possible T-Norm operators.

• Minimum: min(a,b)=a ٨ b• Algebraic product: ab• Bounded product: 0 ٧ (a+b-1)

C(x) = AB = S(A(x), B(x)) where S is called S-norm operator.

• It is also called T-conorm• Some of the T-conorm operators• Maximum: S(a,b) = max(a,b)• Algebraic sum: a+b-ab• Bounded sum: = 1 ٨(a+b)

Linguistic variable, linguistic term

• Linguistic variable:A linguistic variable is a variable whose values are sentences in a natural or artificial language.

• For example, the values of the fuzzy variable height could be tall, very tall, very very tall, somewhat tall, not very tall, tall but not very tall, quite tall, more or less tall.

• Tall is a linguistic value or primary term• Hedges are very, more or less so on

• If age is a linguistic variable then its term set is

• T(age) = { young, not young, very young, not very young,…… middle aged, not middle aged,… old, not old, very old, more or less old, not very old,…not very young and not very old,…}.

Concentration and dilation of linguistic values

• If A is a linguistic value then operation concentration is defined by CON(A) = A2, and dilation is defined by DIL(A) = A0.5. Using these operations we can generate linguistic hedges as shown in following examples.

• ● more or less old = DIL(old);• ● extremely old = CON(CON(CON(old))).

Fuzzy Rules

• Fuzzy rules are useful for modeling human thinking, perception and judgment.

• A fuzzy if-then rule is of the form “If x is A then y is B” where A and B are linguistic values defined by fuzzy sets on universes of discourse X and Y, respectively.

• “x is A” is called antecedent and “y is B” is called consequent.

Examples, for such a rule are

• ● If pressure is high, then volume is small.• ● If the road is slippery, then driving is

dangerous.• ● If the fruit is ripe, then it is soft.

Binary fuzzy relation• A binary fuzzy relation is a fuzzy set in X ×

Y which maps each element in X × Y to a membership value between 0 and 1. If X and Y are two universes of discourse, then

• R = {((x,y), R(x, y)) | (x,y) Є X × Y } is a binary fuzzy relation in X × Y.

• X × Y indicates cartesian product of X and Y

• The fuzzy rule “If x is A then y is B” may be abbreviated as A→ B and is interpreted as A × B.

• A fuzzy if then rule may be defined (Mamdani) as a binary fuzzy relation R on the product space X × Y.

• R = A→ B = A × B =∫X×Y A(x) T-norm B(y)/ (x,y).

References

• Zadeh, L. (1965), "Fuzzy sets", Information and Control, 8: 338-353

• Jang J.S.R., (1997): ANFIS architecture. In: Neuro-fuzzy and Soft Computing (J.S. Jang, C.-T. Sun, E. Mizutani, Eds.), Prentice Hall, New Jersey